Keywords builtins

In this lesson, we explore the fundamental distinctions between Bash built-in commands and shell keywords. Built-ins execute inside the shell without spawning extra processes, whereas keywords are parsed tokens that implement control structures and logic flow.

Shell Built-ins vs Keywords

Shell built-ins (like echo, cd, or [ ]) are implemented directly within Bash, complete with flags and documentation via help or man. Keywords (like if, for, or [[ ]]) are special words the shell interpreter parses to direct execution order and logic.

Single vs Double Square Brackets

Single brackets ([ ]) are a built-in alias for the test command. Double brackets ([[ ]]) are keywords with enhanced features and direct parsing by Bash.

builtin-sample.sh

#!/bin/bash
if [ 2 -eq 2 ]; then
    echo "two equals two"
fi

$ ./builtin-sample.sh
two equals two

keyword-sample.sh

#!/bin/bash
if [[ 2 -eq 2 ]]; then
    echo "two equals two"
fi

$ ./keyword-sample.sh
two equals two

[ vs test

Under the hood, [ ] is just the test built-in. You can invoke either name interchangeably:

$ test 2 -eq 2 && echo "two equals two"
two equals two

View its documentation with:

$ man test

Since [[ ]] has no external binary, Bash handles it entirely as a keyword.

Built-in vs Keyword Evaluations

Single brackets interpret < as a redirection operator:

$ [ 1 < 2 ] && echo "1 is less than 2"
-bash: 2: No such file or directory

Double brackets support < as a comparison operator:

$ [[ 5 < 7 ]] && echo "5 is less than 7"
5 is less than 7

Advanced Double Bracket Features

Double brackets unlock logical grouping, glob patterns, and regular expressions:

$ [[ 3 -eq 3 && (2 -eq 2 && 1 -eq 1) ]] && echo "Parentheses can be used"
Parentheses can be used

$ name="Bob Doe"
$ [[ $name = *o* ]] && echo "Patterns can be used"
Patterns can be used

$ name="Bob Doe"
$ [[ $name =~ B.*Doe ]] && echo "Regular expressions can be used"
Regular expressions can be used

Advantages and Disadvantages

Single Square Brackets ([ ])

Advantages:

• Portable across POSIX-compliant shells
• Standard conditional syntax

Disadvantages:

• Limited to basic comparisons
• No pattern matching or grouping

Double Square Brackets ([[ ]])

Advantages:

• Extended conditionals (regex, globs, grouping)
• Safer string comparisons

Disadvantages:

• Not POSIX compliant
• Bash-specific feature

Summary: Built-ins vs Keywords

• Shell built-ins run inside Bash without forking.
• Keywords are parsed tokens controlling flow or behavior.
• Neither built-ins nor keywords spawn external processes.

Certain keywords (e.g., time) act like directives rather than forming explicit control structures. Armed with these distinctions, you can choose the most efficient and appropriate syntax for your Bash scripts.

Links and References

• Bash Reference Manual
• POSIX Shell Syntax
• Bash Conditional Expression Documentation